Matronics Email Lists :: View topic

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Trim switchs

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

JimT

Joined: 27 Feb 2006
Posts: 22
Location: Auburn, ME

Posted: Sun May 06, 2007 4:46 pm Post subject: Trim switchs

Hello folks



Just exploring an option.  Awhile before I started lurking here I bought an EXP bus.   yes , yes I know, I wouldn't buy it today, but it seemed a good idea at the time.

so I am setting up the trim motor with controls  on the pilots joystick and the throttle, so the right guy can  acess trim if desired.  The usual drill is a rely to avoid  direct shorts when opposite conditions are set on the two switches.  Shouldn't happen often, will happen.



Problem is I don't know much ( anything?) about the characteristics of the PTC breakers in the EXP Bus. Do they trip fast enough to protect 20 ga wire? The manual states they will reset in 10-15 seconds after load removal , and can be tripped thousands of times without  damage.  Solid state devices usually are quite reliable when used within the stated limits.  Would depending on the breaker be more reliable then  the DPDT relay?

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

retasker(at)optonline.net
Guest

Posted: Sun May 06, 2007 5:47 pm Post subject: Trim switchs

The PTC breaker will switch almost instantly and will easily protect the 

20 ga. wire - that is its whole purpose in life.



The way a PTC works is that under normal operating conditions it is a 

very low resistance device.  As long as the current through the device 

is kept below the trip current nothing happens (except current is 

allowed to flow).  As soon as the current gets too high, the device 

starts to heat up.  When it heats up its resistance increases.  Of 

course, when the resistance increases, it heats up faster  and the 

resistance goes up more.  This all happens in milliseconds if you have a 

significant overload.  The resistance goes up orders of magnitude which 

then limits the current to a few milliamps through even a short 

circuit.  As long as the short is there, the high resistance creates 

enough heat to keep it at a high resistance.  As soon as the short is 

removed, the PTC cools down and the resistance goes very low again, 

allowing normal currents to flow.



However, I might use the DPDT relay anyway.  For two reasons:



One reason is that, while the PTC easily protects the wire, it may not 

do such a good job protecting the switch contacts.  Depending on what 

switch you use, you might be overloading the contacts for too long.  I 

suppose it depends on what switch you use and how many times you expect 

to have a direct short between the stick and throttle controls whether 

it is a problem or not.



The second reason is:  As long as this is the only device on that 

particular PTC you would probably be okay as noted above.  However, if 

you are sharing the PTC with other devices, tripping it by shorting the 

switches would shut everything else on the protected circuit down.  

Unfortunately, the PTC will now no longer automatically reset when the 

short is removed - the other devices are still trying to draw current 

and will keep the PTC set!  You will have to shut off all other loads on 

that particular PTC so it can reset and them turn them all back on.



Dick Tasker



JimT wrote:



 	  Quote: 	
		  



Hello folks



Just exploring an option.  Awhile before I started lurking here I bought an EXP bus.   yes , yes I know, I wouldn't buy it today, but it seemed a good idea at the time.

so I am setting up the trim motor with controls  on the pilots joystick and the throttle, so the right guy can  acess trim if desired.  The usual drill is a rely to avoid  direct shorts when opposite conditions are set on the two switches.  Shouldn't happen often, will happen.



Problem is I don't know much ( anything?) about the characteristics of the PTC breakers in the EXP Bus. Do they trip fast enough to protect 20 ga wire? The manual states they will reset in 10-15 seconds after load removal , and can be tripped thousands of times without  damage.  Solid state devices usually are quite reliable when used within the stated limits.  Would depending on the breaker be more reliable then  the DPDT relay?





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=111236#111236



  




	


-- 

Please Note: 

No trees were destroyed in the sending of this message.  We do concede, however,

that a significant number of electrons may have been temporarily inconvenienced.

--

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum